Colonizing responses of Maruca testulalis (geyer) (Lepidoptera: pyralidae) to different cowpea cultivars in relation to their Resistance/Susceptibility

Abstract

A comparison of the resistance/susceptibility levels of ten cowpea cultivars to the pod borer Maruca testulalis revealed that VITA l was the most susceptible. TVU 946 most resistant and VITA 5, chola local and kamboinse local moderately resistant. In all the cultivars which were compared, flowers and pods suffered the heaviest damage compared to the stems. Colonizing responses of M. testulalis namely oviposition, larval orientation, feeding, utilization of ingested food, larval development and adult fecundity were studied on three cultivars namely VITA 1 (susceptible), VITA 5 (moderately resistant) and TVU 946 (resistant). The adults showed ovipositional nonpreference for TvU 946 and VITA 5 when these cultivars were available to the moths as a choice against VITA l • All the cultivars were equally accepted for oviposition under no choice situation. Majority of the eggs were laid on the leaves. Settling sites for 1st instar larvae on the cowpea plant in a decreasing order of preference were: flowers> terminal shoots = flower buds. The first instar larvae were capable of reaching any part of the cowpea plant either by walking or by means of silken thread which they produced and used for swinging from one plant part to the other. However, the 4th instar larvae were not very efficient in climbing vertical objects since a majority of them lost grip and dropped down when climbing up the peduncles of the cowpea plant, especially those of TVu 946, which were very long and erect . As a result, pods of the cowpea plant (Tvu 946 cultivar) which were raised above the canopy were less infested byM. testulalis larvae. Attraction and arrest/stay of 1st instar larvae by the resistant Tvu 946 and VITA 5 was lower than those by VITA 1. The volatiles serving as olfactoary stimuli from the leaves and flowers and the chloroform and n-hexane extracts of these parts were attractive to M. testulalis larvae. However, the attractancy of the volatile and extracts of VITA 1 to the larvae was higher than that from TVU 946 and VITA 5. The larvae were also attracted by high humidity and their speed and rate of arrival on the cowpea plant were enhanced on a moist ground than on a dry surface . Nonpreference for larval feeding coupled with a shorter feeding duration was also observed on Tvu 946 plant parts as compared to VITA 1. Growth and development of the larvae on stems of TVU 946 and VITA 5 were lower than those on VITA 1 because TVu 946 was consumed in very small quantities while food from VITA 5 was poorly converted into body tissues. Although quantity of food ingested from pods was similar for VITA 1 and TVu 946 and even higher on VITA 5, digestibility of the food from Tvu 946 was very low compared to VITA 1. This led to a higher larval mortality on Tvu 946 than on VITA 1. Conversion of the food VITA 5 into body tissue was very low and this led to a lower pupal weight on VITA 5 pods. Antibiosis was therefore considered to be partly involved in the resistance of TVu 946 and VITA 5 pods and stems.